The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.
(1) Field of the Invention
The present invention generally relates to a method and system for determining underwater effective sound velocity.
(2) Description of the Prior Art
In many target tracking scenarios, it is often necessary to determine the effective sound velocity (xe2x80x9cESVxe2x80x9d) between the source of sound pulses and a target located at a particular depth. The ESV between the two points is the ratio of the slant range and the transit time. The transit time is defined as the time required for sound traveling between the source and the target. The slant range is defined as the straight line distance between the source and the target. Thus, in order to effect a hydrophone survey or ship tracking procedure, the transit time between a pinger and a hydrophone is typically recorded and converted into slant range by multiplying the transit time by the corresponding ESV. The accuracy of the calculated slant range depends upon the accuracy of the ESV. The ESV between two underwater points is difficult to determine because the speed of sound is not homogeneous in the ocean medium.
One conventional method for determining the ESV is the ray tracing method. Typical ray tracing methods focus on the determination of the total travel time and horizontal displacement from a source of sound pulses located at a first depth to a target located at a second depth that is greater than the first depth. In addition to the known quantities, i.e. the water depths of the source of sound pulses and the target, additional information needed to carry out the ray tracing method are the associated: (i) the sound velocity profile (xe2x80x9cSVPxe2x80x9d), (ii) the horizontal distance between the source of sound pulses and the target, and (iii) incident angle at the starting depth, i.e., the depth of the source of sound pulses. However, in survey or tracking related applications wherein it is desired to determine the ESV between two arbitrary underwater points, the incident angle is not known. Since only the elevation angle is known, use of the ray tracing algorithm to determine the ESV requires a number of iterations wherein each succeeding iteration commences with an improved guess on the incident angle. Thus, it can be seen that ray tracing algorithms are too time consuming. Furthermore, ray tracing methods are only valid for high frequency signals, i.e., when the wavelength is significantly less than (i) the water depth, (ii) the distance from a source sound pulses to the target, and (iii) the distance from target to receiver.
Another conventional method for determining the effective sound velocity ESV is the Taylor series expansion formulas. The information needed for the Taylor series method is: (i) the depth of the source of sound pulses, (ii) the depth of the target, (iii) the horizontal distance between the source of sound pulses and the target, and (iii) the elevation angle. There is no need for iterations in the Taylor series method. The Taylor series expansion formula remains valid for different SVPs and for arbitrary depth segments and is excellent for estimating ESVs under large elevation angles. However, for small elevation angles, the Taylor series formula fails to converge thereby yielding erroneous results. Including higher order Taylor series terms will not improve the solution. On the contrary, adding higher order terms often worsens the solution for small elevation angles. Another disadvantage of the Taylor series method is that since numerical integrations are required, the Taylor series method is relatively more computation-intensive than other conventional methods. A further disadvantage is that the results produced by the Taylor series method must be numerical compared with the results of the detailed ray-tracing method in order to determine the solution accuracy of the Taylor series method, especially for smaller elevation angles.
Another conventional method for determining ESV is an empirical formula method. Although the empirical formula method is probably the most time-economic approach, the derived empirical formula will often be restricted to one or two parameters. Even limiting the applications to only one particular SVP environment such that the horizontal distance is fixed, deriving a formula of ESV as a function of elevation angle and underwater depths of the source of sound pulses and the target is still a relatively difficult curve-fitting problem. The most appealing feature of the empirical approach is its computational efficiency. However, the task of determining the empirical coefficients is laborious. For example, when a change in the SVP or depth of the first underwater point (i.e. the source of sound energy) is imposed, one has to go through a lengthy process of recalculating the empirical coefficients. Furthermore, the accuracy of the formula needs to be verified again.
Therefore, it is an object of the present invention to provide a new and improved method and system for determining the ESV between two underwater points.
It is another object of the present invention to provide a new and improved method and system for determining the ESV between two underwater points that is relatively less time consuming than conventional methods.
It is a further object of the present invention to provide a new and improved method and system for determining the ESV between two underwater points that may be used with high and low frequency signals.
Other objects and advantages of the present invention will be apparent to one of ordinary skill in the art in light of the ensuing description of the present invention.
The present invention is directed to a method and system for determining the effective sound velocity between underwater points. The system utilizes a device such as a computer or microprocessor for determining the effective sound velocity between the underwater points. The following information is fed into the device: (i) the sound velocity profile from a source of sound energy located at an initial depth to a predetermined final target depth, (ii) a predetermined set of grazing angles, (iii) a predetermined number of target depths between the initial depth and the final target depth, and (iv) a predetermined uniform set of elevation angles. A corresponding elevation angle and an effective sound velocity value is calculated for each grazing angle and target depth. The calculated elevation angles are scanned to locate a pair of calculated elevation angles which correspond to a pair of successive grazing angles and a particular target depth wherein the particular elevation angle of the uniform set is between the pair of calculated elevation angles. The calculated effective sound velocity values corresponding to each elevation angle of the pair of calculated elevation angles are interpolated to produce an interpolated effective sound velocity. The system comprises means for repeating these steps for each target depth fed into the device. Parameters associated with an actual target are then fed into the device. These parameters comprise an actual target depth and an actual target elevation angle. The interpolated effective sound velocities are then scanned to locate an interpolated effective sound velocity that corresponds to an elevation angle of the uniform set and a target depth that matches the actual target elevation angle and actual target depth, respectively. If such an interpolated effective sound velocity is located, it will be the effective sound velocity between the source of sound energy and the actual target.
If it is determined that there is a target depth that matches the actual target depth and there is no elevation angle of the uniform set that matches the actual target elevation angle, then the uniform set of elevation angles is scanned to find a pair of successive elevation angles such that the actual target elevation angle is between the pair of successive elevation angles of the uniform set. The effective sound velocities corresponding to the pair of successive elevation angles of the uniform set and the target depth are then interpolated to produce an effective sound velocity between the source of sound energy and the actual target.
If it is determined that there is no target depth that matches the actual target depth and there is an elevation angle of the uniform set that matches the actual target elevation angle, then the target depths are scanned to locate a pair of successive target depths such that the actual target depth is between the pair of successive target depths. The effective sound velocities corresponding to the pair of successive target depths and the elevation angle of the uniform set are then interpolated to produce an effective sound velocity between the source of sound energy and the actual target.
If it is determined that there is no elevation angle of the uniform set that matches the actual target elevation angle and there is no target depth that matches the actual target depth, then the target depths and the elevation angles of the uniform set are scanned to locate (i) a pair of successive target depths such that the actual target depth is between the pair of successive target depths, and (ii) a pair of successive elevation angles of the uniform set such that the actual target elevation angle is between the pair of successive elevation angles of the uniform set. The effective sound velocities corresponding to the pair of successive target depths and the pair of successive elevation angles of the uniform set are interpolated to produce an effective sound velocity between the source of sound energy and the actual target.